Ultrasonic energy has become a useful tool in solving a variety of problems in industrial and commercial applications. Examples of such applications include medical uses such as the imaging of body tissue or of the flow of blood, and signal processing uses such as narrowband filtering of electrical signals. Many of the new and inventive uses of ultrasonic energy require a greater degree of electronic feedback and control.
Feedback is needed to determine if the ultrasonic energy being generated and delivered to a transducer is at the correct frequency and energy level. Getting quick feedback on the ultrasonic energy being delivered is a problem when the electrical characteristics of the transducer, such as the resonant frequency, dynamically change. In order to maintain optimum energy transfer through the transducer, the ultrasonic energy driving the transducer needs to match these electrical characteristics. Quick control of the characteristics of the ultrasonic energy, such as frequency and energy level, is needed to react to feedback about less than optimum energy transfer. Furthermore, delivering energy to the transducer at the incorrect frequency can undesirably heat the transducer and be destructive to the transducer. Therefore, electronic systems providing such ultrasonic energy to excite an ultrasonic transducer need to be highly efficient, quick reacting, and provide near real-time feedback when less than optimum energy transfer conditions occur.
A particular use of ultrasonic energy is modifying the viscosity of a liquid, thereby modifying the flow rate of the liquid as it passes through an orifice by effecting the rheology of the liquid. This ultrasonic viscosity modification (UVM) is the subject of another U.S. patent application submitted on behalf of the present inventors and is disclosed in U.S. patent application Ser. No. 08/477,689 filed on Jun. 7, 1995, which is hereby incorporated by reference. The UVM patent application describes a system whereby ultrasonic energy is applied to excite a liquid which results in an increase in the flow rate of the liquid. The increase in flow rate of the liquid after excitation with ultrasonic energy advantageously varies from 25 percent to 200 percent when compared to flow rates before excitation.
More specifically, the UVM patent application discloses a system and method for modifying the flow rate of a pressurized liquid, such as a molten thermoplastic polymer. As the pressurized liquid passes through an orifice and is shaped into threadlines or fibers, ultrasonic energy is applied to excite the pressurized liquid. By applying ultrasonic energy to the pressurized liquid, the viscosity of the pressurized liquid is changed in the vicinity of the orifice, thereby increasing the flow rate of the liquid.
The system disclosed in the UVM patent application includes a die housing with a chamber. The chamber is adapted to receive the pressurized liquid from an inlet of the die housing and to expel the pressurized liquid from an exit orifice. A mechanism for applying ultrasonic energy to the pressurized liquid (such as an ultrasonic horn) is located within the chamber. The ultrasonic horn is adapted to apply ultrasonic energy directly to the pressurized liquid within the chamber but not to the die housing. The die housing remains stationary. The application of ultrasonic energy to the liquid is accomplished via a vibrating mechanism in contact with the liquid and a waveguide coupled to the end of the vibrating mechanism (ultrasonic horn).
The system disclosed in the UVM patent application functions by supplying the pressurized liquid to the die housing, exciting the pressurized liquid in the vicinity of the exit orifice with ultrasonic energy without applying ultrasonic energy to the die housing itself, and passing the pressurized liquid out of the chamber through the exit orifice. Thus, the system changes the viscosity of the pressurized liquid by applying ultrasonic energy to the liquid which increases the flow rate of the liquid.
Referring again to the UVM patent application, an ultrasonic power converter and an analog power meter are used to provide a drive signal to a vibrating mechanism or transducer. The described ultrasonic power converter and the analog power meter (drive electronics) can (1) generate the correct alternating current (ac) frequency of the drive signal in order to match the transducer impedance, (2) deliver a specific energy level of the drive signal to the transducer, and (3) sense changes in the transducer's resonant frequency so that the frequency and energy level of the drive signal may be adjusted. It would be advantageous if such drive electronics for controlling the transducer provided highly efficient, quick reacting, near real-time control of the drive signal and near real-time feedback when less than optimum energy transfer conditions occur.
First, it would be advantageous to quickly track and correct for changes in a transducer's resonant frequency. It would be advantageous to do so because optimum energy transfer through the transducer can be maintained by supplying the drive signal at the transducer's resonant frequency. In general, ultrasonic transducers are used to convert electrical energy into mechanical energy. Most transducers are reciprocal in that they will also convert the mechanical energy back into electrical energy. Typically, an ultrasonic transducer is manufactured for a specific resonant frequency due to physical dimensions. However, the resonant frequency of the ultrasonic transducer may shift in response to the changes in temperature and loading of the transducer. The shift in resonant frequency leads to electrical impedance matching problems and less than ideal energy transduction.
To solve these problems, certain systems drive ultrasonic transducers and correct for misalignment of the drive signal with respect to the changing resonant frequency of the transducer. For example, a Model 48A100 ultrasonic welding system designed and marketed by the Dukane Corporation, St. Charles, Ill. uses an oscillator to generate the drive signal applied to the transducer. The Model 48A100 system detects the power output delivered to the transducer, conditions the detected power signal, and correspondingly readjusts the frequency of the oscillator. In this manner, the system senses the shift in resonant frequency of the transducer and corrects for misalignment of the drive signal. However, the system is not capable of sensing the changing resonant frequency of the transducer within a period of the drive signal. Furthermore the system does not provide any operator feedback or telemetry signals corresponding to the rheological properties of the medium excited by the transducer.
It would also be advantageous to provide a smaller, more efficient electronic system for driving and controlling an ultrasonic transducer. Prior art electronic systems, such as in ultrasonic welding applications, use low efficiency designs implemented with large discrete linear power amplifiers. Typical energy transfer efficiencies for such prior electronic systems are approximately thirty percent. When the energy level needed to drive an ultrasonic transducer is large, efficiency in driving the ultrasonic transducer may become a concern for heat dissipation and energy conservation reasons. Thus, it is advantageous to drive and control an ultrasonic transducer using smaller, more energy efficient electronics that are less costly than prior art electronic systems.
Finally, it would be advantageous to precisely adjust the flow of liquid as the liquid flows through an orifice. The previously mentioned UVM patent application describes a fuel injector apparatus having a nozzle orifice and utilizing an ultrasonic transducer for injecting liquid fuel into a cylinder of an internal combustion engine. Ultrasonic energy is applied to the pressurized liquid fuel as it passes through the nozzle orifice to enhance the atomization of the liquid fuel and to facilitate deeper penetration into the engine cylinder before combustion occurs. As described, the application of ultrasonic energy acts as a flow adjustment on the flow of liquid fuel through the nozzle orifice. It would be advantageous to precisely control liquid flow in an injection orifice with an ultrasonic transducer to enhance internal combustion engine performance during cold starts and warm-up conditions. Furthermore, more control of fuel flow is desired in order to reduce pollution from unexpended fuel expelled from the engine cylinder. Thus, there is a need for an apparatus and method of using an ultrasonic transducer to provide more control of the flow rate of a liquid.
In summary, there is a need for an improved method and apparatus to drive an ultrasonic transducer so as to (1) quickly control the drive signal applied to the ultrasonic transducer, (2) provide useful and timely feedback about the resonant frequency of the ultrasonic transducer, (3) provide telemetry signals corresponding to the rheological properties of the medium in contact with the transducer, (4) drive and control the ultrasonic transducer with electronics that are smaller, weigh less, and cost less than prior electronic systems, and (5) provide more control of the flow rate of liquid using the ultrasonic transducer.